Optically sensitive materials such as optical sensors and the human eye are used to detect light because they contain components which are sensitive to light energy. For example, an optoelectronic sensor, such as an "electric eye" or a focal plane array, produces a voltage when light falls upon a sensing material within the sensor. However, such sensors can be "blinded" by overexposure to high intensity light, just as the human eye becomes blinded if it is exposed to overly intense light. In each case, exposure to high intensity light can temporarily or even permanently destroy the ability of the optically sensitive material to react to light.
There are two approaches to protecting such optically sensitive materials against blinding by overexposure to high intensity light. In the first, or "active" approach, when the electronic circuitry of the optically sensitive device detects a harmfully high intensity of light, it operates a mechanism to interpose a physical barrier between the light source and the optically sensitive material. The second, or "passive" approach, is employed when the onset of the harmful high-intensity light is so fast that no active system has the time to respond. In a passive protection system, a barrier is formed directly in response to the incident light. One such system, or device, is called an "optical limiter" and is characterized by the behavior that as the intensity of incident light increases, the intensity of light transmitted through the system also increases up to a saturation level. Above the saturation level, the transmitted light intensity remains substantially constant even with increasing intensity of incident light.
Optical limiters are devices one class of which comprises materials which exhibit reverse saturable absorption, that is, materials which absorb more light as the incident intensity is increased. These devices must employ lenses to focus the light into the reverse saturable materials. The higher intensity of the focussed light is required to activate the optical limiter when the intensity of the incident light is weak and insufficient to activate the optical limiter but still sufficient to damage the eye or sensor.
U.S. Pat. No. 5,080,469, issued Jan. 14, 1992, and assigned to the same assignee as the present application, describes a process for making a solid phase optical limiter with a graded distribution of reverse saturable absorber molecules in the limiter. The disclosed process involves fusing a plurality of wafers together, each having a different molecular concentration of the reverse saturable absorber in a host polymer, such as polymethyl methacrylate (PMMA). The fusing is accomplished by applying a thin film of unpolymerized PMMA to each face of the wafer, stacking the wafers, and allowing the film to cure. In this manner, a gradient, or non-uniform concentration, in the reverse saturable absorber is obtained along the optic axis, which produces a self-protecting effect. The optic axis is that axis parallel to the incident light on the optical limiter.
While the teachings of this patent are certainly adequate, the fabrication process is not as convenient as one might desire, the process is expensive, and the process is not scalable to commercially viable production. Accordingly, an improved process for making optical limiters with graded distribution is required.
A new reverse saturable absorber comprises buckminsterfullerenes, such as C.sub.60 or C.sub.70, which may be formed as a thin film or incorporated in a host material, such as PMMA, as disclosed in U.S. Pat. No. 5,172,278, also assigned to the same assignee as the present application. Buckminsterfullerenes are carbon-only cages (or carbonaceous hollow cage molecules), so named because of their resemblance to geodesic shapes described by Buckminster Fuller.
A need remains for a simple process for fabricating an optical limiter having a graded distribution of a reverse saturable material. Such a process would utilize the advantages of the prior art teachings while avoiding most, if not all, their disadvantages.